Question 38 QMED02 - Electrician-Refrigerating Engineer

The Correct Answer is D. ### Explanation for Option D (Correct Answer) A thermal-magnetic circuit breaker utilizes two primary tripping mechanisms: 1. **Thermal Trip (Overload Protection):** Provided by a bimetallic strip that heats up and bends, tripping the mechanism for sustained overloads (the *inverse time* characteristic). 2. **Magnetic Trip (Short-Circuit/Fault Protection):** Provided by an electromagnet that trips the mechanism instantly when the current reaches a very high magnitude (the *instantaneous* characteristic). The breaker is rated at $\text{500 A}$ continuous load. Standard industrial circuit breakers (especially those protecting large equipment like a $\text{300 kW}$ alternator) have magnetic trip settings that are typically between 5 to 10 times the continuous rating. * **Breaker Rating:** $\text{500 A}$ * **Typical Instantaneous Trip Range:** $\text{500 A} \times 5 = \text{2,500 A}$ to $\text{500 A} \times 10 = \text{5,000 A}$ (or even slightly higher depending on the specific type). An instantaneous current draw of $\text{5,000 amperes}$ represents a severe fault condition (like a major short circuit) that is at or above the upper limit of the magnetic trip setting for a $\text{500 A}$ breaker. The magnetic trip mechanism is designed to operate immediately (instantaneously) to protect the alternator and wiring from catastrophic damage under such fault conditions. Therefore, this condition will trip the breaker. ### Explanation for Incorrect Options **A) Sustained current draw of 450 amperes for 2 hours** This current ($\text{450 A}$) is below the full continuous load rating ($\text{500 A}$). Since the breaker is designed to handle $\text{500 A}$ continuously without tripping, $\text{450 A}$ is considered normal operation and will not cause the thermal element to trip, regardless of duration. **B) Sustained current draw of 500 amperes for 10 minutes** $\text{500 A}$ is the full continuous rating. By definition, a circuit breaker rated for $\text{500 A}$ must carry $\text{500 A}$ indefinitely (or at least for extended periods) without tripping. While loading the breaker exactly at its limit, it is unlikely to trip after only $\text{10 minutes}$ unless the ambient temperature is extremely high or the breaker is already faulty. Tripping typically occurs only when the current significantly exceeds the rating (e.g., $\text{125}\%$ to $\text{150}\%$ overload) for a period inversely proportional to the excess current. **C) Momentary current draw of 1000 amperes for 3 seconds** $\text{1000 A}$ is $200\%$ of the $\text{500 A}$ rating. While this is a significant overload, $\text{3 seconds}$ is usually too short a time for the thermal element to heat up sufficiently to trip the breaker, especially if the breaker uses standard time-delay characteristics. Furthermore, $\text{1000 A}$ is generally below the magnetic trip threshold (which starts around $\text{2,500 A}$ for a $\text{500 A}$ breaker). Therefore, the breaker is designed to ride through transient overloads like motor starting currents, and $\text{1000 A}$ for $\text{3 seconds}$ is unlikely to cause a trip.